An Ultimate Collection of Royal Defense throws magic and mayhem into the tower defence genre, now on EU PSN for £7.39 from 8Floor, published by 4HIT. It comes with all the content from the mobile and PC game crammed into the one release, and still only 130MB.
The game may be three years old, but has been extensively patched and given additional content, and every game should have at least one tower game on their system (the original Fieldrunners is my personal favourite).
4HIT has helped bring a bunch of mobile ports to the Vita, including Mahjongg, Solitaire, Alien Shooter and other defence-style games. Would be cool if they'd show a little more ambition with some edgier titles, but I guess they know their niche.
Update: Also out from 4HIT is a Doodle God bundle for £5.79, allowing players to create their own worlds by mixing chemcials. I have no idea if that's fun or not.